Too Much Sugar

Sue went in for her yearly physical exam. She was nervous about what the doctor would say, since she had put on a little weight in the last year (well, actually more than a little). She had a urinalysis done while she was there just to check on some things. That was when her doctor informed her that her urine sugar was positive and that she might have diabetes mellitus.

Why It Matters

Testing for sugar in the urine is a quick and non-invasive way of screening for diabetes, a disorder in which the body does not use glucose correctly. Today we have modern lab tests for this parameter. Back in the “old days”, things were not so easy. In medieval times, the urine was tested by sitting it out on a window sill and seeing if the bees were attracted to it. Another unpleasant test was to taste the urine to see if it was sweet.

Credit: Jessica Merz

Source: http://www.flickr.com/photos/jessicafm/187457292/

License: CC BY-NC 3.0

Diabetics must regularly check their blood glucose level to make sure it stays in the safe range
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The first chemical tests for urinary glucose were based on simple organic chemistry. Glucose is what is known as an aldohexose – a six-carbon compound with an aldehyde group at carbon one. This compound is known as a reducing sugar because it can be oxidized to form a carboxylic acid while reducing a metal cation to a lower oxidation number.

Two very similar tests for reducing sugars are Benedict’s reagent and Fehling’s reagent/ Both are based on a reaction with Cu2+, but in two different buffers. Glucose is oxidized to gluconic acid while the blue Cu2+ solution becomes a red precipitate when the copper is reduced to Cu+.

Another common test was the Tollens’ test. In this method, a solution of [Ag(NH3)2]+ (a silver-ammonium complex is mixed with the urine. The silver ions are reduced to metallic silver, forming a grey precipitate in the bottom of the test tube. Many textbooks talked about seeing a silver mirror, but that almost never happened.

Modern methods utilize a test strip that can measure the amounts of several constituents. Glucose is measured using the glucose oxidase enzyme method. The enzyme oxidizes glucose to the carboxylic acid while forming hydrogen peroxide. This compound then reacts with the reduced form of a dye (colorless). In acid, the oxidized form of the dye turns pink, indicating a positive test.

Watch a video at the link below to see the performance of a Benedict’s test: